Apparatuses for determining insulation resistance are required for safety reasons and are often integrated on the input side of a photovoltaic inverter in order to ensure insulation monitoring of the PV generator that is connected there. Apparatuses for determining insulation resistance are known, for example, from EP 0 654 673 B1, according to which, for each pulsed voltage value of the measurement voltage, the time profile of the transient response of the measurement current until a transient state is reached is monitored by supplying a plurality of pulsed AC voltages to the grid that is to be monitored. The measurement value difference of two consecutive measurement values is used to determine the insulation resistance between grid and ground.
If photovoltaic modules that are susceptible to degradation effects, for example PID (potential induced degradation) in the case of crystalline modules or TCO (transparent conductive oxide) corrosion in the case of thin-film modules, are connected to the photovoltaic inverter, devices for shifting the electric potential with respect to ground are connected in order to prevent said effects. Devices of this kind are well known; DE 20 2006 008 936 U1 from the applicant is mentioned as an example. This method is used particularly in large photovoltaic installations having an ungrounded PV generator and galvanic isolation from the grounded AC grid. The device for shifting the electric potential with respect to ground can in this case shift the negative pole or the positive pole of the PV generator to a defined potential with respect to ground, wherein the PV generator is typically shifted to such an extent that the entire PV generator (including the negative pole thereof) is positive with respect to ground potential. Within the context of the disclosure presented here, the term “shift” does not in any way imply a direction indication of the potential reference; here, it only means that the device for shifting the electric potential with respect to ground specifies a fixed, defined reference-ground potential, usually by means of a voltage source.
DE 10 2011 051 954 A1 describes an apparatus for a photovoltaic installation having a bias generation device for generating a bias at an inverter of the photovoltaic installation. The apparatus is intended to prevent TCO corrosion and polarization effects. DE 10 2011 055 220 A1 relates to a method for connecting a transformerless inverter in a solar power plant having an offset potential center point.
Problems arise when a device for shifting the electric potential with respect to ground at a PV generator is operated simultaneously in parallel with an apparatus for determining insulation resistance at a PV generator by means of an active method as described above. The device for shifting the electric potential with respect to ground maintains the potential of the PV generator with respect to ground, while the apparatus for determining insulation resistance attempts to shift the potential of the PV generator with respect to ground. Since for the most part the devices for shifting the electric potential have a higher efficiency than the apparatus for determining insulation resistance, the apparatus for determining insulation resistance is “overruled” and an excessively low, but at least distorted, insulation resistance value is measured.
For this reason, in practice, the device for shifting the electric potential with respect to ground at a PV generator is temporarily disconnected in order to determine the insulation resistance of said PV generator by means of the apparatus for determining insulation resistance at a PV generator. This means that the insulation resistance determination is not carried out continuously, but instead is carried out only at specific times. This state is unsatisfactory because, on the one hand, the PV generator should permanently be at the potential prescribed by the device for shifting the electric potential with respect to ground in order to prevent degradation effects, but it should also be possible to determine the insulation resistance at any time for safety reasons.